Various types of apparatus are known for varying the operational speed of devices driven by a prime mover such as an electric motor. Spring loaded pulleys have been used, for example, for varying speeds by using motor bases which are adapted to have a linear motion. This motion is usually accomplished by turning a handle affixed to a threaded rod which drives a screw to move the motor forward and backward, causing the pulley sheave to open and close, which action is also accompanied by a change in the effective pitch diameter of the pulley. The threaded rod can be turned by a motorized reduction gear; however, this requires a means to stop and start the motor at the end of its travel, as well as a means to reverse the direction of travel.
A chief limitation of known prior art variable speed drives is the relatively sophisticated means employed to achieve their objective and as a result are relatively complex, costly and difficult to implement. This is particularly true with respect to direct current drives, fluid drives, PIV drives and eddy current clutches. While such drives have particular utilization where very accurate speed control is required over a relatively large speed range, fan and pump speed control do not require ranges of speed which are as great nor do they require a high degree of accuracy.